Study on a Unified Fracture Mechanics Parameter Controlling Crack Growth Behavior and its Evaluation Method

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The crack energy density was proposed as a parameter which may describe all kinds of crack behavior in a unified form regardless of fracture types such as brittle-ductile fracture,fatigue fracture and creep fracture. In this research project,the way of expressing crack growth rate by the crack energy density and the evaluation method of thes parameter by numerical analysis are studied and,moreover,the initiation condition of creep crack growth is studied based on the crack energy density by using a newly-developed equipment for observing the deformation around a crack tip. The results are as follows;1. By considering the crack growth rates of ductile,fatigue and creep cracks from the standpoint of crack energy density,it is made clear that their growth rates can be described in a unified form by the increments of crack energy density at crack tip and fracture resistance expressed by crack energy density per unit time(per one cycle in case of fatigue) and the gradient of crack energy density distribution.2. The discontinuous crack model proposed as a model,which enablesus to evaluate the distribution of crack energy density as well as the value at crack tip,is extended so as to be able to deal with a creep problem and a new method to evaluate the crack energy density in this model by a path-independent integral is developed. The availability of the model is confirmed through the analyses of a standard problem.3. An equipment for observing the deformation around a crack tip out of the furnace is developed and the experiment of creep crack growth is carried out. The value of crack energy density evaluated by the method above at the time when a crack starts to grow suggests the existence of the critical value of crack energy density peculiar to the material.